when I replaced the vavle stem seals on my 91 560 SEl, thesea re the steps that I followed:
1. Used a whistler to locate DTC. This method allowed me to turn the crank at the bottom rather than on top, and did not have to remove the fan, etc.
2. I did not use Air pressure. At TDC, the valve will not fall. It only have a fraction of an inch space to fall into.
3. When a cyclinder is at TDC on the left cylinder, there is a corresponding cylinder on the right that is also on TDC, so I worked on 2 cylinders per TDC setting.
4. A good spring compressor makes removing and re-installing keepers less difficult. Keepers are hard to remove and hard to re-install if your spring compressor keeps on slipping...